1. Field of the Invention
The present invention relates to a falling film type heat exchanger tube, such as a heat exchanger tube for a falling film evaporator for performing a heat exchange between a falling film of refrigerant (water) formed on an external surface of a tube and a water flowing inside this tube to evaporate this refrigerant, and a heat exchanger tube for a falling film absorber for performing a heat exchange between an absorption liquid film dripped or dispersed on an external surface of a tube and a fluid flowing inside this tube to cool the absorption liquid.
2. Description of the Prior Art
Conventionally, an absorption type heat exchanger such as an absorption type chiller has been used in such a way that the inside of the heat exchanger is kept in a vacuum state and a refrigerant on the outer surface of the tube is evaporated at a low temperature to obtain cold water in the tube by extracting an evaporation latent heat from the water in the tube. This cold water obtained is used for an air-conditioner or the like.
According to this heat exchanger, an absorber and an evaporator are accommodated together inside one body. In order to obtain evaporation continuously, a refrigerant vapor generated by the evaporator is absorbed into an absorption liquid dispersed on the surface of a heat exchanger tube, and the inside of the body is maintained at a constant degree of vacuum. Accordingly, in order to improve the refrigeration capacity of an absorption type chiller, it is necessary to increase the quantity of the refrigerant vapor generated in the evaporator and to increase the absorption quantity or the absorption capacity. Improving the performance of the heat exchanger tube is the most effective means for increasing the absorption capacity. For this purpose, the applicant of the present invention proposed a heat exchanger tube having formed independent fins by providing grooves and hills extending in a tube axial direction on an external surface of the tube (Japanese Patent Application Laid-Open Public No. 9-113066).
Further, according to a falling film type evaporator such as an absorption type water cooler, there has been performed a heat exchange between a refrigerant that flows down on an external peripheral surface of a heat exchanger tube and a liquid such as water that flows through inside this tube, thereby to cool the water within the tube. The refrigerant which flows down on the heat exchanger tube spreads out the surface of the heat exchanger tube, and is then evaporated at a low pressure while taking heat, at the same time, from a surface of the heat exchanger tube, thereby to cool the water inside the heat exchanger tube.
As described above, according to the falling film type heat exchanger tube for an evaporator, a refrigerant such as pure water, is dispersed on the external surface of the tube and cold water is passed through inside the tube. Then, a liquid film of the refrigerant is formed on the external surface of the tube. When this refrigerant evaporates, the cold water flowing inside the tube is cooled. In this case, at the time when the refrigerant wet and spread on the surface of the heat exchanger tube evaporates, the latent heat of vaporization is deprived from the heat transfer surface. Therefore, in order to efficiently cool the water inside the tube, it is necessary to increase as far as possible the contact area between the heat exchanger tube and the refrigerant, that is, the area of the heat transfer surface (external surface of the tube).
For providing a falling film type heat exchanger tube that meets this requirement, the applicant of the present invention proposed a heat exchanger tube provided with a large number of fins on the external surface of the tube (Japanese Patent Application Laid-open Public No. 7-71889). According to this conventional heat exchanger tube, there are provided fins extending in a direction to be orthogonal with or in a spiral fashion with respect to a tube axial direction, on the external surface of the tube, and there are also provided grooves on the tops of the fins along with these fins. Further, there are provided concavities crossing an upper half portion of each fin in predetermined pitches. An angle formed between both side walls of each groove is within a range from 70 to 150.degree..
This heat exchanger tube has an advantage that the spreading property of the refrigerant is excellent, with a large surface area of heat transfer, resulting in a superior heat transfer performance to that of the prior art.
The above-explained conventional heat exchanger tube for an absorber described in Japanese Patent Application Laid-Open Public No. 9-113066 has concavities on the external surface of the tube at the rate of 3 to 25 (concavities/tube circumferential length). Therefore, this tube has sufficient spreading property of the absorption liquid in a tube circumferential direction. However, on the other hand, in the tube axial direction, the spreading property is so poor that the absorption liquid leaves the surface of the tube before the absorption liquid absorbs the vapor generated by the evaporator, with a result of performance reduction.
The above-mentioned conventional heat exchanger tube for an evaporator described in Japanese Patent Application Laid-open Public No. 7-71889 has achieved the initially intended object. However, the heat transfer performance of this tube has come insufficient as a heat exchanger tube for an evaporator for which higher performance has been required increasingly in recent years, as explained below. According to this conventional heat exchanger tube, grooves are provided in a longitudinal direction of fins, and the upper half portion of each fin is divided into two in a Y shape as viewed from the cross section orthogonal with the longitudinal direction of the fins, with the division angle of each fin being within a range from 70 to 150.degree.. Since, these divided portions close the grooves formed between the fins in the end, a spreading property of the refrigerant to the grooves between the fins is poor and thick liquid film is formed, thus lowering the evaporation performance.
Further, the fins are disconnected at concavities extending in a direction orthogonal with the longitudinal direction of the fins. Since, the concavities have a smaller deepness than the height of the fins, thus providing insufficient spreading property of the refrigerant in the tube axial direction. As a result, a liquid film is formed in a large thickness; which lowers the evaporation performance.